Four Of A Kind-Chapter 158: [3.60] Taqueria Guadalupe

I flipped open the cover when he gave the signal.

Twenty problems. Multiple choice for the first ten, free response for the last ten.

Find the derivative of f(x) = 3x² + 5x - 7.

Easy. Power rule. Bring down the exponent, multiply, reduce the power by one.

6x + 5.

Good. Next.

If f(x) = sin(2x), find f’(x).

Chain rule. The derivative of sin is cos, but you have to multiply by the derivative of the inside function.

2cos(2x).

Still with it. Next one.

Find the derivative of (x² + 1)(3x - 4) using the product rule.

Product rule. First times derivative of second, plus second times derivative of first.

Except I couldn’t remember which was first and which was second.

Was it (x² + 1) · 3 + (3x - 4) · 2x?

Or was it backwards?

I stared at the problem.

The numbers didn’t move like Cassidy said hers did. They just sat there, mocking me for not knowing something I’d done a hundred times before.

Breathe.

Product rule is just distribution with extra steps. If you can’t remember the formula, derive it from first principles.

I worked through the algebra. Expanded everything. Combined like terms.

Got 9x² - 8x + 3.

That felt right. Probably. Maybe.

I circled it and moved on.

The next seven problems blurred together. Some I knew instantly. Others required me to work backward from the answer choices, plugging in numbers until something clicked. By problem ten, my hand was cramping and I’d used the "when in doubt, pick C" strategy twice.

Not ideal.

The free response section was worse.

Find all critical points of f(x) = x³ - 6x² + 9x + 2.

Critical points meant taking the derivative and setting it equal to zero. Find where the slope was flat.

f’(x) = 3x² - 12x + 9.

Set that equal to zero.

3x² - 12x + 9 = 0.

Divide everything by 3.

x² - 4x + 3 = 0.

Factor.

(x - 3)(x - 1) = 0.

So x = 3 or x = 1.

Good. That was right. I knew that was right.

Next problem.

A ladder 10 feet long leans against a wall. The bottom slides away from the wall at 2 feet per second. How fast is the top of the ladder sliding down when the bottom is 6 feet from the wall?

Related rates.

The problem I’d spent two hours teaching Harlow yesterday while she kept getting distracted by her phone buzzing with Fashion Club emergencies.

I set up the equation. Drew the triangle. Applied the Pythagorean theorem.

Then I forgot whether I was supposed to differentiate with respect to time or with respect to x.

My pencil hovered.

I could picture Harlow’s face when she finally understood it. The way her eyes lit up and she said "OH!" so loud that Mrs. Chen shushed us from the reference desk.

I could picture Cassidy color-coding her graph paper in the library, tongue between her teeth as she concentrated.

I could picture Vivienne adjusting my collar with her fingers lingering just slightly too long.

I could picture Sabrina’s hand in my hair, her voice soft as she said sweet dreams.

What I couldn’t picture was the actual derivative of this related rates problem.

"Forty minutes remaining."

I was screwed.

Alright. New strategy. When you can’t remember the process, remember the principle.

Related rates meant everything changed together. The bottom moved, so the top moved. The rate of change connected them.

I worked through it slower. Step by step. Like I’d taught Cassidy.

Got an answer of -3/4 feet per second.

Negative because it was sliding down. That made sense.

I moved on to the next problem, and the next, and the next.

Some I nailed. Others felt like I was guessing with extra steps.

By the time Chen called time, I had answers for everything. Whether those answers were correct remained a mystery I didn’t want to solve.

"Pass your tests forward."

I handed mine to the girl in front of me.

Felix turned around again and made a throat-slitting gesture.

I gave him a thumbs up that I absolutely did not feel.

"Alright, get out of here. I’ll see you Monday."

The exodus was immediate.

Felix grabbed my arm in the hallway. "Tacos. Now. I need to emotionally eat my feelings about that disaster, and you’re coming with me."

"I should—"

"No. Whatever you’re about to say, the answer is no. You look like you’re about to pass out. When’s the last time you had a real lunch?"

I couldn’t actually remember.

"Exactly. Come on. My Range Rover has heated seats and good speakers. We’re getting al pastor and you’re gonna tell me why you’ve been acting like someone replaced you with a sad robot."

"I don’t—"

"Nope. Already decided. Let’s go."

He pulled me toward the parking lot before I could argue further.

His Range Rover sat in its usual spot, gleaming white and probably worth more than my entire apartment building. Felix unlocked it with his key fob and climbed into the driver’s seat.

I got in the passenger side and immediately sank into leather that probably cost more than my monthly rent.

"Buckle up. Chelsea is calling our names."

He pulled out of the lot with the confidence of someone who’d never had to parallel park in his life, merging into traffic while simultaneously changing the radio station to something with way too much bass.

Manhattan passed by in its usual chaos. Taxis honking, delivery trucks double-parked, pedestrians jaywalking with the kind of death wish that came from living here too long.

"Is it the Valentine sisters?" Felix asked. "Because you’ve been weird ever since you started working for them."

"Define weird."

"You check your phone every thirty seconds. You zone out during conversations. Yesterday you called me Vivienne."

"I did not."

"You literally did. I said ’want chips?’ and you said ’not now, Vivienne, I’m thinking.’"

Oh.

That was bad.

"It’s just stress," I said. "The job’s demanding."

"Uh-huh. And I’m the next Emperor of Japan."

He turned onto a side street and pulled into a spot that definitely wasn’t legal but Felix had never cared about parking laws in his entire life.

"We’re here. Taqueria Guadalupe. Best al pastor in the city. My dad took me here after I failed my driver’s test the first time."

"You failed your driver’s test?"

"Twice. Don’t judge." He cut the engine. "Come on. Tacos wait for no man."

The place was small. Maybe six tables total, with a counter in the back and a kitchen visible through a window. The smell hit me immediately. Roasted pork, fresh cilantro, lime, corn tortillas.

My stomach reminded me I’d skipped breakfast.

An older woman with graying hair and kind eyes greeted Felix by name. "Félix! ¿Cómo estás, mijo?"

"Bien, Señora Rosa! This is my friend Isaiah. He’s having a day."

She looked at me with that specific maternal assessment that saw right through everything. "Ay, pobrecito. Sit, sit. I’ll bring you something good."

We took a corner table.

Felix leaned back in his chair. "Okay. Real talk. What’s actually going on?"

I considered lying.

Then I considered that Felix had been my best friend for three years and had never once asked for anything in return.

"My mom texted me."

His eyebrows shot up. "Your mom. The one who—"

"Yeah."

"Holy shit. What’d she say?"

"That she’s been thinking about us. That she’s sorry. That she wants to explain."

"And you said?"

"Nothing. I blocked her number."

Felix processed this. "Damn."

"Yeah."

"How do you feel about it?"

"Like I want to throw my phone into the Hudson and never think about her again."

"That’s fair. Also probably expensive."

"Hence the blocking instead."

Señora Rosa appeared with two plates piled high with tacos. Al pastor, cilantro, onions, lime wedges. Probably a week’s worth of sodium.

Looked perfect.

"Eat," she commanded. "You’re too skinny."

I picked up a taco and bit in.

The flavor exploded across my tongue. Savory pork, bright lime, fresh cilantro. The kind of food that reminded you that life had good moments between all the garbage.

Felix was already two tacos deep. "So your mom. You gonna tell Iris?"

"She already knows. Figured it out this morning."

"Smart kid."

"Too smart. She asked if I was okay approximately seventeen times."

"Were you?"

"No."

"Are you now?"

I considered the question while eating my second taco.

"Not really. But the tacos help."

"See? I told you. Universal cure." He grabbed another one from his plate. "What about the Valentines? They know?"

"Vivienne figured it out yesterday. Didn’t even have to tell her."

"That’s... kind of scary actually. How’d she know?"

"She said I looked like I was punishing myself. Which apparently is something she recognizes because she does it too."

Felix paused mid-bite. "Wait. Vivienne Valentine opened up to you? About personal stuff?"

"Kind of."

"Bro. That’s huge. She doesn’t talk to anyone. Like, anyone. The girl runs half the school and nobody knows a single real thing about her."

"She’s seventeen and managing a billion-dollar brand while maintaining a 4.0. Can’t blame her for being closed off."

"Except she opened up to you."

"It wasn’t—"

"Dude." Felix set down his taco and looked at me seriously. "I know you think you’re just the help or whatever. But those girls don’t act like that around normal staff."

"Your point?"

"My point is you’re not just their assistant anymore. If you ever were."

I finished my third taco instead of responding.

Felix wasn’t wrong. I knew he wasn’t wrong.